1. Field of Invention
This invention relates to gasification systems for the production of electricity from biomass, such as shredded bark, wood chips, sawdust, sludges and other carbonaceous fuels or feedstocks. More particularly, the present invention relates to an improved method of operating a parallel entrained bed pyrolysis unit with improved circulation and reduced erosion of system components.
2. Description of Related Art
Biomass gasification systems have been developed which are useful for the production of electrical power in remote areas or in areas wherein a large amount of agricultural biomass waste is produced. Current biomass gasification systems generally rely on combustion of a portion of the biomass feedstock to provide the heat required for gasification of the remainder of the biomass feedstock. However, the combustion of a portion of the raw biomass stream for heat production can significantly reduce the overall efficiency of the gasifier system. It has also proven advantageous to utilize the waste carbonaceous char produced in the gasification as a fuel source for generating heat in a combustor. Since the char is basically a waste product from the gasifier, its consumption in the combustor has less of an adverse effect on the system efficiency than is seen in systems wherein a portion of the raw biomass is used as a combustor fuel source.
U.S. Pat. No. 4,828,581 to Feldmann et al, describes an exemplary gasifier system for the production of fuel grade gas from carbonaceous fuels using very high biomass throughputs in a fluidized bed gasifier operating at low inlet gas velocities. The process described in Feldmann et al. uses a combustor to heat a bed of fluidized sand, which is directed to a gasifier wherein the heated sand serves as a heat source for the pyrolysis of the biomass material. Unlike prior systems, the system of Feldmann et al. relies on the entrainment of char in a flow of sand from the gasifier outlet to allow operation at an advantageously low inlet velocity of as low as 0.5 ft/sec but with a biomass throughput from 500 to 4400 lbs/ft2-hr. The Feldman et al. system is suited to the production of a medium BTU gas which may be used as a fuel source for the production of electricity in a standard gas fired boiler/turbine system.
One of the problems commonly associated with the use of such fluidized bed gasifier systems is the erosion of the piping comprising the systems by the circulating sand used to transfer heat within the gasifier system. This problem has been found to be especially severe at bends in the system piping, wherein the circulating sand can severely erode the piping. In severe cases, this erosion can shorten the lifetime of the gasifier system and may lead to catastrophic failure of the piping.
In fluidized bed systems wherein sand is used as a heat transfer medium from a combustor to a gasifier, it is necessary to minimize or eliminate the leakage of oxygen containing gases from the combustor into the gasifier. Contamination of the gasifier with oxygen results in the undesirable formation of carbon dioxide and water from the CO and H2 end products of the gasification reaction, lowering the efficiency of gasification. However, it has proven difficult in prior systems to develop a method whereby the sand may be transported from the combustor to the gasifier and back while maintaining an air tight seal to prevent entry of oxygen into the gasifier.
In some instances, depending upon the nature of the feedstock used, these prior systems have also experienced problems resulting from the agglomeration of the ash, sand, and char mixture, and subsequent blockage of flow through the system. At the high operating temperatures of gasifier systems, at least a portion of the agglomeration of ash is the result of the partial melting of the ash constituents. It would clearly be desirable to develop a method of reducing or eliminating the agglomeration of the ash, sand and char mixture.
Accordingly, it is an object of the present invention to provide an improved method of directing the flow of sand through a parallel entrained bed pyrolysis system whereby erosion of system components is minimized.
It is another object of the present invention to provide an improved method of allowing the flow of sand and char in a fluidized bed pyrolysis system while maintaining and air tight seal between the gasifier and the combustor components of the system.
It is yet another object of the present invention to provide an improved method of reducing or preventing the agglomeration of ash, sand and char in a fluidized bed pyrolysis system.
The process system according to this invention relates to improvements to a parallel entrainment fluidized bed gasifier system. A first aspect of the present invention relates to a method for reducing ash agglomeration in a parallel entrainment fluidized bed gasifier/combustor system. A carbonaceous feedstock is provided and supplemented with a quantity of MgO prior to introduction into the gasifier combustor system. Upon gasification and combustion, the MgO alters the eutectic of the resultant ash to raise the melting point and substantially reduce the agglomeration of ash and sand which results from partial ash melting at high temperatures.
A second aspect of the present invention relates to an apparatus and method for reducing erosion at piping bends in fluidized particulate piping systems which utilizes sand retention cavities positioned at the piping bends to receive and retain a portion of the fluidized particulate. The retained fluidized particulate serves as an ablatable buffer to protect the surface of the piping bends from erosion by the flow of particulate impacting the wall.
A third aspect of the present invention relates to an apparatus and method for facilitating the flow of sand and char fragments from a first compartment to a second compartment while minimizing the flow of gases between the first and second compartments. A surger chamber is provided for receiving a flow of sand and char fragments from the first compartment. The surger chamber includes an inlet nozzle disposed to deposit the sand and char mixture into the lower portion of the surger chamber. An outlet is disposed above the point at which the nozzle deposits sand and char mixture into the surger chamber, such that the outlet is disposed to allow the gravitationally driven flow of sand and char from the surger chamber to the second compartment. Thus, when operating, the surger chamber maintains a quantity non-fluidized sand and char disposed between then inlet nozzle and the outlet, which acts to maintain a substantially gas resistant seal between the first and second compartments.